1. Field of the Invention
The present invention relates to a magnesium based alloys article (hereinafter also referred to as Mg alloy article) and a method of producing the Mg alloy article and more particularly, an Mg alloy article which is produced in such a manner that a Zn layer which is excellent in adhesion is applied on an Mg alloy base material and a Cu/Ni plating layer and a coating film are applied thereon and a method of producing the Mg alloy article with good efficiency.
2. Description of the Related Art
Mg alloy has been known as a metal whose specific gravity is lightest among metals in practical use and applications thereof have been studied in various fields which have an object of producing an light weight article. Speaking in a detailed manner, Mg alloy has been used in many aspects of industrial fields such as plural kinds of covers mounted on a transport vehicle, for example an automobile, home electronics products of a handy type, for example a video-camera, a personal computer of a note book type and a machine or equipment having a movable part and the usage thereof has been increasing without any pause in recent years.
In general, since Mg alloy is chemically active very much and its corrosion in an ordinary environment progresses, its surface is in advance protected with a surface treatment before use. As conventional surface treatments, there are named: an anodic treatment represented by DOW method, HAE method and the like, a chemical conversion such as chromate and in addition resin coating on these surface treatment layer.
While Mg alloy articles applied with such surface treatments are improved in anticorrosion, abrasion resistances, such as surface injury resistance, shock resistance and sliding abrasion resistance, are insufficient and therefore practical application of the alloy is still to be improved as a whole. Accordingly, there has been a great amount of desire for improvements on the abrasion resistances. In an aspect of designing, the alloy article has a poor appearance after an anodic treatment or a chromate treatment and besides there arises a new fault that a metallic glossiness is reduced even when the poor appearance has been improved by resin coating.
Moreover, these surface treatment methods use a lot of harmful materials such as chromic acid and the like and thereby there arises a problem that a liquid-waste treatment cost is increased.
As can be seen from the above description, the conventionally adopted anodic treatment, chromate treatment, coating method and the like are still insufficient in order that Mg alloy can have various functions such as the abrasion resistances, designing and the like in addition to anticorrosion and a variety of simple methods have been proposed for substitution therefor.
For example, in Publications of Unexamined Japanese Patent Application No. Hei 4-311575 and Examined Japanese Patent Application No. Sho 61-36596, a underlayer preparation method as a DOW method is disclosed in which after an Mg alloy base material is subjected to a zinc immersion process, a copper cyanide plating is conducted on the surface. Fundamental process steps comprise: degrease.fwdarw.water cleaning.fwdarw.acid cleaning.fwdarw.activation.fwdarw.water cleaning.fwdarw.a zinc immersion process.fwdarw.water cleaning.fwdarw.copper cyanide plating (the aforementioned process is an underlayer preparation treatment).fwdarw.water cleaning.fwdarw.a desired plating treatment. Among the steps, the steps from degrease to activation are steps of cleaning and activating the surface of the Mg alloy base material and a step of zinc immersion thereafter is a step of zinc underlayer treatment by coating the activated surface of the Mg alloy base material with zinc. In such a manner, the above mentioned method is to coat the zinc underlayer with a copper plating layer to its fullness by performing copper cyanide plating after suppressing an active reaction of the surface of Mg alloy through forming a zinc underlayer. When such a series of underlayer treatment steps as this is applied, a desired plating treatment which follows can be performed in various conditions without being affected by environmental factors such as pH, temperature and the like. However, copper cyanide which is used in these methods is harmful and new costs for facilities and waste-liquid treatment occur. However, if copper plating is tried to be performed by other plating methods than copper cyanide plating in the above mentioned series of steps in a underlayer preparation method, there occurs a problem that good plating adhesion cannot be achieved.
While other than the above mentioned methods, there has been proposed a method in which after a zinc underlayer is formed by a zinc immersion process, a separate plating is applied, adhesion is different according to kinds of Mg alloy and kinds of treatment batch, and lacks stability, in addition other problems, plating defects such as plating swell, arise in a great extent and abnormalities are apt to occur in a plating portion. Moreover, even if a plated surface has a beautiful appearance immediately after plating, such a phenomenon as plating swell occurs by being heated in a baking finish in coating or the like has been observed.
As other plating methods, there has been proposed a method in which electroless Ni plating is directly performed on AZ 91 material (a nomenclature in ASTM standard). This method does not use an zinc immersion process and a copper cyanide plating process and forms a desired plating layer in one process, whereby a problem of liquid-waste treatment is eliminated and improvement on adhesion is to be achieved. However, this method has a problem that it can be applied to only a specific kind of alloy and lacks versatility of application.